Generally, an outer race rotation bearing is configured such that an annular space defined between an inner race and an outer race is closed at both axial ends of the annular space by a pair of seals mounted on the respective end portions of the inner peripheral surface of the outer race, a plurality of rolling elements are arranged between the inner race and the outer race, lubricant such as grease is sealed in the annular space by the seals, the inner race is fixed, and the outer race rotates. Conventionally, in order to extend the service life of the bearing, the measure has been taken that the inner diameter of the outer race is enlarged so as to increase the volume of the annular space, thereby increasing the amount of lubricant to be sealed in the annular space.
However, in some cases, such an outer race rotation bearing shortens rather than extends its service life by enlarging the inner diameter of the outer race and thus increasing the amount of lubricant to be sealed in the annular space. The reason is explained below using a ball bearing as an example. As illustrated in FIG. 6(a), generally, a seal 52 mounted on each end portion of an outer race 51 includes an annular mounting portion 53 formed of a rubber, and the mounting portion 53 is fitted in an annular groove 54 formed in the inner peripheral surface of the outer race 51. In this arrangement, if the inner diameter of the outer ring 51 is large, grease G to which centrifugal force is applied strongly presses the seal 52 axially outwardly. As a result thereof, in some cases, as illustrated in FIG. 6(b), the entire seal 52 is slightly moved axially outwardly due to the elastic deformation of the mounting portion 53, thereby causing grease G to leak through the gap generated between the seal mounting portion 53 and the annular groove 54.
Furthermore, if the amount of grease G to be sealed is increased, the stirring resistance of grease G also becomes large and thus the temperature of the bearing rises. As a result thereof, since the rubber of which the seal 53 is formed deteriorates faster than usual, grease G is likely to leak in this respect, too.
As countermeasures against such grease leakage as described above, generally, seal mounting portions are formed of a rubber which is less likely to be deteriorated by heat, and grease which generates little heat is chosen. Also, various proposals are made for preventing the temperature of a bearing from rising. For example, Japanese Unexamined Patent Application Publication No. 2009-174588 discloses that a pulley made of a resin is fitted on an outer race such that both end surfaces of the outer race and both end portions of the outer peripheral surface of the outer race are not covered by the pulley, thereby enabling more heat to be released from the bearing.
However, in particular, in an outer race rotation bearing used under high-speed conditions in which the rotation speed of the bearing nearly reaches its limit, such as a pulley bearing for use in the engine of automobiles, since a large centrifugal force tends to act on grease and also the temperature of the bearing tends to rise, the above-described conventional countermeasures are not sufficient for dealing with such problems, and thus the service life of the bearing tends to be shortened due to grease leakage.